Liquid-crystal cells using FLC have drawn increasing attention as elements that can be used as displays in watches, hand-held calculators, various OA (office automation) equipment and television sets. A liquid-crystal cell using FLC is manufactured by a process that comprises the steps of providing a pair of glass substrates each equipped with and electrode and an alignment film, bonding the periphery of each substrate with a sealant so as to leave a central hollow portion that is to be filled with a liquid crystal, and filling the cell under high-temperature and high-vacuum condition with a liquid crystal that is injected through a preliminarily formed inlet. Another way to produce a liquid-crystal cell comprises providing a sealant in the periphery of a glass substrate equipped with an electrode and an alignment film, coating the sealant-surrounded portion with a liquid crystal to a predetermined thickness by a suitable means such as coating or printing, superposing the other glass substrate on the first glass substrate, and compressing them together with the sealant heated to an elevated temperature.
Various proposals about FLC compositions have been made with a view to improving the display capability of liquid-crystal cells. The present inventors previously found that satisfactory displays having high contrast and free from afterimage could be obtained by adding to FLC compositions those oleophilic compounds, or compounds that were rendered lipophilic, which were either cyclic or possessed of structures capable of forming cycles or cages, with particularly good results being achieved by adding compounds that worked as complex forming ligands with respect to ions. An example of such compounds is at least one oleophilic compound represented by the general formula (I): ##STR1## where a, b, c, d, e and f are each independently an integer of 0-4, provided that a+b+c+d+e+f is greater than 7; --A--, --B--, --C--, --D--, --E-- and --F-- which may be the same or different are each a group selected from among --CH.sub.2 --, --CHR'--, --CH.dbd.CH--, ##STR2## (where R is an alkyl having 1-12 carbon atoms; R' is an alkyl having 1-12 carbon atoms) the group --CH.sub.2 -- may be substituted by --O--, --COO--, --OCO--, phenyl, Cl, F or CN.
It is preferred to use macrocyclic compounds of the general formula (I) where a, b, c, d, e, f, R and R' have the same meanings as defined above, --B--, --C--, --E--, and --F-- are each a group --CH.sub.2, and --A-- and --D-- which may be the same of different are each a group selected from among ##STR3##
It is particularly preferred to use olephilic compounds of the general formulas (I) where a, b, c, d, e and f are each independently an integer of 0-3; --B--, --C--, --E-- and --F-- are each a group --CH.sub.2 --; --A-- and --D-- which may be the same or different are each group a selected from among ##STR4## (where R is an alkyl having 1-12 carbon atoms and R' is an alkyl having 1-12 carbon atoms or a phenyl).
In a more preferred embodiment, groups --A-- and --D-- are selected from among ##STR5##
An amide that serves as an inophore and that is represented by the following general formula (II) can also be added to FLC composition: ##STR6## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each independently an alkyl having 1-15 carbon atoms at least one --CH.sub.2 group in which may be substituted by --COO--, --CO-- or --O--, a cyclohexyl, a phenyl or a benzyl; X is an alkylene having 2-9 carbon atoms, with one or two non-adjacent --CH.sub.2 -- groups being optionally substituted by --O--, with two adjacent CH.sub.2 groups being optionally substituted by 1,2-phenylene or 1,2-cyclohexylene, with two adjacent --CH.sub.2 -- groups being optionally substituted by CH(CH.sub.3)--CH(CH.sub.3)--, and with the hydrogen atoms in the groups CH.sub.2 being optionally substituted by R.sup.5 or R.sup.6, provided that R.sup.5 is an alkyl having 1-15 carbon atoms and R.sup.6 is an alkyl having 1-15 carbon atoms or CH.sub.2 --O--CH.sub.2 --CO--NR.sup.1 R.sup.2.
It is preferred to use a ferroelectric liquid-crystal mixture containing an amide of the formula (II), where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each independently an alkyl having 1-15 carbon atoms at least one --CH.sub.2 -- group in which is optionally substituted by --COO-- or --O--, a cyclohexyl or a phenyl; X is an alkylene having 2-9 carbon atoms, with one or two non-adjacent --CH.sub.2 -- groups being optionally substituted by --O--, with two adjacent CH.sub.2 groups being optionally substituted by 1,2-phenylene or 1,2-cyclohexylene, with two adjacent --CH.sub.2 -- groups being optionally substituted by CH(CH.sub.3)--CH(CH.sub.3)--, and with the hydrogen atoms in the groups CH.sub.2 being optionally substituted by R.sup.5 or R.sup.6, provided that R.sup.5 and R.sup.6 are each independently an alkyl having 1-15 carbon atoms.
Particularly preferred is an amide of the formula (II), where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each independently an alkyl having 1-15 carbon atoms, in which one --CH.sub.2 -- group is optionally substituted by --COO--, or a cyclohexyl; X is an alkylene having 2-9 carbon atoms, with one or two non-adjacent --CH.sub.2 -- groups being optionally substituted by --O--, with two adjacent CH.sub.2 groups being optionally substituted by 1,2-phenylene, and with two adjacent --CH.sub.2 -- groups being optionally substituted by --CH(CH.sub.3)--CH(CH.sub.3)--.
Particularly preferred is an amide of the formula (II) where --X-- is a group selected from among the following: ##STR7## where R.sup.1 -R.sup.6 have the same meanings as defined above.
Most preferably, --X-- is a group selected from among the following: ##STR8##
In principle, a broad range of inophores are suitable for use in liquid-crystal mixtures but amides of the formula (II) are particularly suitable for suppressing a twisted state.
Compounds represented by the following general formula (III) or (IV) are also useful: ##STR9## where --Z-- is --O-- or --S--; m and n are each an integer greater than 0, provided that m+n is 2-6: --X.sup.1 -- and --X.sup.2 -- which may be the same or different each represents ##STR10## when --X.sup.1 -- and --X.sup.2 -- are taken together, ##STR11## (where --R is an alkyl or alkanoyl having 1-15 carbon atoms, a phenyl, a benzyl or a benzoyl; and I is 1 or 2); ##STR12## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each independently ##STR13## p, q, r and s are each independently an integer of 2-4, provided that p+q+r+s is 88-16.
It is preferred to use a compound of the general formula (III) where m and n are each an integer greater than 0, provided that m+n is 2-4; --X.sup.1 -- and --X.sup.2 -- which may be the same or different each represents ##STR14## when --X.sup.1 -- and --X.sup.2 -- are taken together, ##STR15## (where --R is an alkyl or alkanoyl having 1-15 carbon atoms, a phenyl, a benzyl or a benzoyl; and I is 1 or 2).
It is also preferred to use a compound of the general formula (II) where ##STR16##
The additive compounds are preferably contained in amounts of 0.01-10 mol %, more preferably 0.1-10 mol %, of the FLC mixture.
When two or more additive compounds are to be present in an FLC mixture, their total amount is in the range of 0.01-10 mol %, preferably 1-10 mol %.
However, the additives described above have low boiling points and hence will evaporate under the high-temperature and high-vacuum conditions that are employed in the already described process for producing liquid-crystal cells. As a result, those additives fail to fulfill their intended function.